How did the Atlantic slave trade begin?

The Atlantic slave trade began in the 15th century on the coast of West Africa, when Portuguese ships started bringing enslaved people back to Europe. Trade to the Americas began in the 16th century and lasted well into the 19th. According to Arlindo Caldeira, in 1444 six caravels docked in Lagos, Portugal, with 235 enslaved men, women and children who had been captured in the Bay of Arguin, in today's Mauritania. Caldeira says historians recognized that episode as the beginning, though ships in Prince Henry's fleet had been bringing enslaved people from West Africa since 1441.

In the Middle Ages, religion rather than race determined who was seen as a legitimate target of slavery. But in the 15th century the Black Sea slave trade was closed off from Europe, and Spain and Portugal replaced that source by importing enslaved people from the conquered Canary Islands and then from mainland Africa. In 1452 Pope Nicholas V issued the bull Dum Diversas, which gave the King of Portugal the right to enslave non-Christians in perpetual slavery.

Then came the Americas. European diseases were shrinking the Indigenous population there, and Europeans were reluctant to emigrate, so the solution was the same one used on the sugar plantations of Madeira and the Canary Islands: enslaved people brought from sub-Saharan Africa.

The scale was vast. According to historian Richard Anderson and colleagues, writing in 2013, the transatlantic trade carried off an estimated 12.5 million captives from Africa, and about 10.7 million survived the voyage to the Americas.